I saw some cheap value RAM in 2 GB dimms at a local store, & noticed that the Crucial was Micron D9FTB (Fatbodies, though not the awesome ones).
And the Buffalo was actually PowerChips, basically the same stuff as most higher end 2x2 GB kits out there today.

I did initial quick testing with Memtest86+, followed by HCI Memtest in Windows.
I find that if you get past 25% in HCI Memtest, it's extremely likely to be very stable, as it's generally at least as sensitive as Memtest86+ is for reporting errors, & takes minutes to find issues vs. hours with Orthos or Prime95.

Buffalo Select 2 GB DDR2-667PowerChips R1C889-AA3G
This or very similar ICs are found in many of the 2x2 GB DDR2 kits out there presently.
They clock well, generally without needing much voltage.

Crucial 2x 2 GB DDR2-667Micron D9FTB
These are Fatbodies (though not the infamous IC of yore)...didn't clock terribly well.
I've seen results with very tight timings with these before, but i wasn't willing to put more than 2.25V into these, so i didn't get spectacular results.

Onto the results...

I didn't bother messing with subtimings, & actually deliberately left them very loose to keep them out of the equation.
Voltages i did mess with, but i didn't spend tons of time trying for lowest possible vdimm.
Just will show the results with reasonable voltage, though i have no doubt the PSC would have run with a lot less voltage, & the D9FTB would run even tighter timings with more voltage.

4-4-3 pretty much always gets as far as 4-4-4 from what i've tested on PSC, & same story here.
Loosened to 5-4-3 & it did DDR2-900.
Basically exactly how things are with my Mushkin actually, though the Mushkin does a bit farther for each of those timings.
DDR2-1067 5-5-3 was pretty much the ceiling; dropping to 5-5-5 did not help getting further.
Very nice for what's essentially crap value RAM IMO :)

Crucial - Micron D9FTBDDR2-800 4-3-3-6 2.25v
DDR2-885 4-4-3-12 2.25v

You never get 4-3-3 @ DDR2-800 on PowerChips, so that was impressive.
Couldn't get past there with those timings though, had to loosen to 4-4-3.
Unfortunately, that was it!
I basically hit a brick wall @ DDR2-900.
Only way to get DDR2-900 stable was to run 5-6-6, which barely got any further either.
I tried numerous different ratios too, & same brick wall.
Seems this stuff is great for tight timings on 2 GB dimms, but only to pretty low speeds...then it just completely craps out.
I will admit CAS 4 till DDR2-880 is nice...you rarely see 2 GB PSC capable of that.
But needing 5-6-6 just to get DDR2-900 stable lol...

Hope this little write-up was worth skimming through.
Or even better if it inspires you to write up your own!